Non-ridging chromium stainless steel

ABSTRACT

NON-RIDGING FERRITIC CHROMLUM STAINLESS STEEL, SUCH AS A.I.S.I. TYPE 430, PRODUCED BY STEPS WHICH COMPRISES SELECTING A CHEMISTRY CONTAINING A MAXIMUM OF 0.01% CARBON, AND PROCESSING THE SELECTED STEEL INTO FINISHED STRIP FORM BY A PROCEDURE WHICH MAY INCLUDE A NORMALIZING TREATMENT ABOVE THE CRITICAL TRANSFORMATION TEMPERATURE.

United States Patent O "ice 3,674,467 NON-RIDGING CHROMIUM STAINLESS STEEL David L. Chalk, Monroe, Ohio, assignor to Armco Steel Corporation, Middletown, Ohio No Drawing. Filed July 23, 1969, Ser. No. 844,213 Int. Cl. C22c 39/16; C21d 9/48 U.S. Cl. 75126 R Claims ABSTRACT OF THE DISCLOSURE Non-ridging ferritic chromium stainless steel, such as A.I.S.I. Type 430, produced by steps which comprise selecting a chemistry containing a maximum of 0.01% carbon, and processing the selected steel into finished strip form by a procedure which may include a normalizing treatment above the critical transformation temperature.

BACKGROUND OF THE INVENTION (1) Field of invention This invention relates to an improved chromium steel and to the method of producing same whereby the finished strip form of said steel exhibits non-ridging characteristics. More specifically, the invention is concerned with improvements on ferritic chromium steels such as A.I.S.I. Type 430, whose general composition limits are as follows:

Percent Chromium 14-18 Carbon (max.) .12 Manganese (max.) 1.0 Silicon (max.) 1.0 Phosphorus (max.) .040 Sulfur (max.) .030 Iron Balance Such steels, when severely drawn into a formed part, are subject to a defect known as ridging. This defect manifests itself in the formation of ridges in the metal parallel to the direction in which the metal was originally rolled, when the meal is drawn. The ridges make the drawn article unsightly and lead to rejections, especially since there is no way of restoring the appearance of a ridged article excepting by an expensive grinding operation; and even this is not always effective.

Ridging is a defect which should be distinguished from the phenomenon of stretcher straining. This problem, which appears as elongated markings on drawn articles, is a function of yield-point elongation and can be alleviated or eliminated by temper rolling the sheet stock be fore drawing the articles. Ridging cannot be so eliminated or minimized. Stretcher straining, moreover, occurs in about the first five percent of the elongation due to drawing, and then disappears, while ridging becomes noticeable first at about this point and increases to a maximum just before the failure of the metal in tension.

One of the more diflicult aspects to the ridging problem lies in translating the severity thereof to a uniform system. At present there is no standard test giving a quantitative measurement of ridging. It has been found that if a strip of the chromium bearing steels is clamped 3,674,467 Patented July 4, 1972 at its ends in a suitable machine and then stretched to a point near but just short of its breaking point, observable ridging will occur in the stretched portion if the metal has any ridging tendency at all. But the degree of ridging must be judged by the eye, so that the test results are, to this extent, subjective.

However, despite this rather unscientific test, a skilled worker and/or steel fabricator can readily recognize the defect and the severity of strain in a drawn part. By selecting a uniform testing procedure, i.e., stretching a sheet sample to a limit short of failure, it is possible to arbitrarily grade the test samples on a ridging scale. Such a scale has been adopted by the assignee of this case, whereby at a first extreme grade 0 exemplifies a material which exhibits no ridging, while at the other end of the scale grade 7 represents material which shows ridging to such a degree as to constitute a very severe defect if applied to drawn parts. Under this grading system a material should have a ridging grade of 1.0 or less to be considered low ridging quality.

(2) Description of prior art In U.S. Patent No. 2,851,384, by J. H. Waxweiler, there is taught a procedure for minimizing the problem of ridging in Type 430 stainless steel. The suggested procedure was based upon the conclusion that ridging is a phenomenon promoted by preferred grain orientation. Hence, a method Was developed to promote the randomization of the; grain orientation. It was determined that a partial randomization of the grains could be accomplished by a phase change or transformation. To achieve this, an austenite promoting composition and treatment were developed. While it may be desirable to review said patent for a more complete understanding thereof, in brief it was found that a Type 430 stainless steel, having an austenite potential of at least 35% and preferably 55%, would exhibit minimum ridging characteristics when heated above the critical transformation temperature.

The normalizing treatment was found effective in producing low ridging quality. However, this procedure involves tight controls in melting the alloy, and was not sufficient to substantially reduce or eliminate the phenomenon without deleteriously afiecting other properties such as hardness and formability.

In a further development, U.S. Patent No. 3,128,211, by the inventor noted above, it was again concluded that low ridging tendencies could be deevloped in Type 430 stainless steel by procedures invvolving control of the grain characteristics of the metal. The factor areas which were evaluated for their effect on the attainment of the desired grain characteristics may be summarized briefly as follows: chemistry, ingot production, hot rolling, and subsequent processing steps.

Production of minimum ridging material Was achieved in part by the formation of an equiaxed fine grain condition in the ingot, the chemistry of which was controlled to include a maximum carbon of .06% and columbium on the order of .05 to .50%.

Coacting with these factors, it was taught that at least the final 50% of the hot rolling should be performed at a temperature below 870 C. T 0 complete the final processing, all steps of which are taught to coact with one another, the hot rolled strip was subjected to a series of cold reductions and anneals, the latter being carried out at temperatures below the critical temperature.

in each of said patents, it should be apparent that the controls are numerous and hence costly. Contrary to this, the present invention contemplates a single control which drastically diminishes or eliminates ridging of ferritic chromium steels when drawn into a fabricated part.

SUMMARY OF THE INVENTION In the practice of this invention, a standard ferritic stainless steel is selected having a chemistry as follows:

Percent Chromium 14-18 Manganese (main) 1 Silicon (max.) 1 Phosphorus (max) .040 Sulfur (max.) .030 Iron Balance The present invention shatters or at least modifies the concept that ridging is associated with the banded microstructure after hot rolling. A more definite concept has now been established by the present development. That is, the removal of carbon from the alloy system results in the elimination of ridging. While a banded micro structure after hot rolling was observed in some low carbon steels of this invention, ridging was not found. On the other hand, in one comparison alloy investigated during the development of this invention ridging was found without banding in the hot rolled structure.

To help demonstrate the improved characteristics of the ferritic chromium steels of this invention, six alloys were prepared and processed to strip form by procedures involving variations in temperatures. The two alloys whose chemistry fall within the teachings of this invention (D and F) showed substantially no ridging regardless of the processing procedure used. The chemical data for the six alloys is listed below.

ALLOY CHEMISTRY A P n' Cr C Mn P S Si percent 6 064 003 016 34 52. 2 .4 061 34 002 017 42 52. 6 .6 057 38 004 015 47 44. l 0 0039 27 002 0064 44 55. l. 4 I2 27 002 010 45 56. 8 5 0019 15 002 0078 24 28. 3

NOTE.A.P."=Austenite potential, based on the Waxweiler fonnula where: A.P.=288 0 plus 350 N plus 22 Ni plus 7.5 Mn18.75 Cr-54 Si plus 338.5.

Ni varied between .42 and .56%.

plus carbon which is limited to a maximum of 0.01%. Optionally, nitrogen can be present in an amount between about .04-09% by weight. It has been determined that the substantial reduction of caubon in the alloy system, from the typical initial value of 0.06%, results in the elimination of ridging.

DETAILED DESCRIPTION OF INVENTION While it is difi'icult to accurately determine the response or changes occurring with thermal treatments of steel alloys which contain a number of alloying agents, one can predict certain changes based upon a simplified system utilizing the primary components of the alloy. For example, a study of the iron-chromium equilibrium system can be used as a basis for the study of Type 430 stainless steel.

It is well known from an analysis of the iron-chromium equilibrium diagram that austenite is present as a high temperature phase at the iron-rich end of the system. The austenite region is further characterized by a two-phase band or region surrounding same, and separating it from the chromium enriched phase of ferrite. This two-phase region is austenite and ferrite. Experience has confirmed the presence of these phases in the typical Type 430 analysis.

With this understanding and background, one major belief that has prevailed in the industry was that ridging of regular Type 43 0 was associated with the banded microstructure, i.e., one composed of alternating bands of ferrite and ferrite plus carbides, resulting from hot rolling in the two-phase region. As a consequence, the previous attempts by the practitioners in the art have been to devise means or procedures to break up the banded structure. The normalizing treatment and the use of colurnbiurn or titanium represent two of the proposed solutions.

Test ingots from each heat Were processed to final strip gauge by each of the following routings:

TABLE I Routing X Routing Y Routing Z 1. Hot roll 1,120 0., 1. Hot roll 955 0., 1. Hot roll 1,120 0.,

3.30 mm. 3.30 mm. 3.30 mm. 2. Box ann. 845 0., 2. Box ann. 845 0., 2. Strip normalize pickle. pickle. 1,1 3. Cold roll, 1.65 mm-.. 3. Cold roll, 1.65 mm.. 3.Boxauk1l1. 845 0.,

10 e. 4. Strip 1gnu. 800 (3., 4. Strip sun. 800 C., 4. Cgld roll, L65 nun.

pic 0. p10 0. 5. Cold roll, .64 mm 5. Cold roll, 134mm.-. 5. Strip aun. 800 0.,

pickle. 6. Strip ann. 800 0., 6. Strip ann. 800 0., 6. Cold roll, .64 mm.

pickle. pickle.

7. Strip ann. 800 0.,

pickle.

Table II shows the mechanical properties and ridging quality of the several alloys treated in the manner above.

TABLE II Percent E m 2% Y.s., uxrrs, 50.8 Hard- Ridging kgJmrn. kg./mm 2 mm. ness B]; grade Sample code:

AX as. e 52. e 20. 5 74. s 1. 4 34.1 52.9 28.0 75.3 1. 5 32.6 54.1 29.0 75.5 1.0 35. 7 51. 9 25. 5 74. 5 2. 4 35.2 51.8 31.5 74.5 0.7 41.8 57.2 24.0 74.5 0. 2 34.2 52.2 25.0 72.0 a. 7 29.4 49.4 31.0 72.0 0. 9 30.0 48.1 2s. 5 71.0 0.1 42.5 50. a 14. 0 78. 5 0 30.3 40.8 24.5 73.0 0 31. 4 50. 5 26. 5 74. 5 0 35.9 53. 2 30.0 75. 5 0.5 34. s 51. 2 a2. 0 74. 2 1. a 36.2 52.9 31.0 74.0 0. 5 25.4 37. 5 35.5 65.0 0.1 25. 2 as. 0 as. 5 65. 0 0 20. 2 as. 0 30. 5 02. 2 *N.G.

{Rough surface with large grains, hence not graded; however, no visible ndging observed.

It may thus be concluded from the above that the removal of carbon from the alloy system, represented by samples D and F, results in the elimination of ridging. That is, these alloys were non-ridging regardless of the process routing. From a microstructure study of the processed samples, it was observed that sample D possessed a banded microstructure after hot rolling. This presumably resulted from hot rolling in the two phased austenite-ferrite region. Note that, while the carbon content was low, the nitrogen content was approximately double in sample D leading to a comparative A.P. value.

In contrasting samples D and F, it should be readily apparent that with the increase in nitrogen in sample D, the austenite potential, strength and hardness were restored or maintained at the levels shown for the remaining alloys. It will be recalled that the A.P. formula given above showed carbon and nitrogen to be strong austenite forming elements.

In any event, the removal of carbon from the alloy system does not mean the remaining properties of the resulting alloy will be sacrificed to achieve the desired results. In fact more avenues are now open to vary the performance of the ferritic chromium stainless steels.

Over and above the dramatic improvements achieved by the removal of carbon, certain varying routing procedures were observed to further enhance the non-riding characteristics of the Type 430 stainless steels. Improvements with some samples were observed with the lower hot rolling temperatures, however, a more general benefit may be seen with a routing which includes a normalizing treatment following the hot rolling. However, as indicated above, at the carbon levels selected for samples D and F the improvement in non-ridging was observed for all routing schedules.

The benefit recognized with the preferred routing procedure will become more apparent as the carbon level of increased above the preferred limit of 006% by weight. Thus, this invention contemplates and includes carbon levels as high as .01 by weight.

I claim:

1. A process of eliminating ridging in a ferritic chromium stainless steel containing about 1418% by 'Weight chromium, upon drawing from strip form, which process comprises selecting a chemistry for said steel with a carbon content no greater than about .006% by weight, and subjecting said steel to a sequence of heat treating and rolling operations, which operations include hot and cold rolling, annealing and normalizing, suitable for reducing said steel to strip form.

2. A process according to claim 1 wherein said processing sequence includes a hot reduction followed by a normalizing treatment.

3. The process according to claim 1 wherein the chemistry is selected to include from .04-.09% by weight nitrogen.

4. A non-rdiging ferritic chromium stainless steel containing about 14-18% by weight chromium, carbon no greater than about .006% by weight, and the balance essentially iron.

5. The uon-ridging ferritic chromium stainless steel according to claim 4 wherein nitrogen is present in an amount from about .04.09% by weight.

References Cited UNITED STATES PATENTS 2,772,992 12/1956 Kiefer et a1. 14812 2,851,384 9/1958 WaXWeiler l26 R 2,965,479 12/1960 Evans 75126 R 3,128,211 4/1964 Waxweiler 14812 3,139,358 6/1964 Graziano 14812 3,490,956 1/1970 Wilton 75126 R L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner US. Cl. X.R. 148--12 

